Life In The Universe

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

What is Life?

Columnist Dave Barry had a very concise definition of life:

Life is what dies when you stomp on it.

This definition has the virtues of being vivid and easily understandable, but lacks a certain je ne sais quoi. Let's see if we can improve on it.

Simple definitions like movement don't work. After all, plants don't move. Growth, reproduction, even metabolism don't work. After all, fire grows, reproduces if a spark lands on something, and feeds. Salt crystals growing out of concentrated brine also grow and feed off their surroundings, and if you drop a crystal into a concentrated solution, it will "reproduce" and grow. Response to stimuli is often cited as a criterion, but lots of living things don't respond to stimuli. On the other hand, fire moves in response to the wind and salt crystals respond to variations in temperature and salt concentration. Quite a few pre-scientific cultures regard fire as alive.

Most of the best and most reliable definitions of life focus on themes like these:

Information Coding

The organism contains some mechanism for passing along details of its structure.

Homeostasis

The organism contains mechanisms for maintaining a constant internal state. Many of these mechanisms involve negative feedback, in which the internal process opposes the external change.

Prebiotic Evolution and DNA

DNA

The hereditary code for humans is contained in the molecule DNA, Deoxyribonucleicacid. The DNA in humans contains a few billion pairs of molecules arranged in a spiralladder form. An atom is about 10^-8 centimeters in diameter, and each rung onthe ladder is a few atoms high, so the total length of human DNA in a single cell is abouta meter. Most cells are microscopic; the DNA fits in such a tiny space because it istightly coiled. The act of uncoiling DNA, splitting it, replicating it and separating thestrands without tangling in such a tiny space is mind-boggling. Ever put your clothes onin a sleeping bag? Imagine taking raw wool or cotton, spinning it into thread, weaving thecloth, sewing it into clothes, and putting them on in the sleeping bag. That's whatreplicating DNA amounts to.

A human body contains about 20 trillion cells. The total length of DNA in a human bodyis thus 20 trillion meters, or twenty billion kilometers, the circumference of the orbitof Pluto. The DNA in a human body would wrap around the Solar System.

Prebiotic Evolution

The basic molecules of organic chemistry are easily made

The classic Miller-Urey experiment of the 1950's showed that it was easy to create organic chemicals like amino acids from inorganic ingredients. Many of these molecules have been detected in interstellar space. Some scientists believe that inorganic precursors of life arrived on Earth ready-made during meteor impacts. The ease of creating organic molecules leads some biologists to believe that life is all but inevitable on any world with suitable physical conditions.

The first self-replicating molecule was almost certainly not DNA

Your cells contain a single-strand self-replicating molecule called RNA (ribonucleic acid). Some viruses contain only RNA. It is much more likely that RNA evolved before DNA, and quite possible that something simpler preceded RNA. The first self-replicating molecules might have been very simple.

DNA assembles from simpler materials all the time

When a DNA molecule splits and replicates, where does the missing half come from? It comes from the simple organic molecules in your cell fluids. When the correct molecule drifts into contact with the DNA, it is attached by the DNA editing molecule. None of this is done consciously. The fact that the mating hald of a DNA molecule is already there is irrelevant; you could pile lumber next to a half-built house with all the necessary tools and blueprints nearby and it would never spontaneously assemble into a house.

Alternative Biologies

Carbon and Water

There are sound physical and chemical reasons for being, as Carl Sagan put it, a"Carbon-Water chauvinist". Carbon can bond to four neighboring atoms and also bond toother carbon atoms, thus making it possible to form a vast array of organic molecules.Silicon, sometimes suggested as an alternative, does not bond easily to itself. (Boronbonds to itself, but only bonds to three neighbors, thus limiting its structuralversatility, and boron is much less common in the universe than carbon.)

Water is a polaror asymmetrical molecule. It stays liquid over a wide range of temperatures. It attractsions easily, making it a good transporter of nutrients, but does not dissolve organicmolecules (so we do not dissolve in our own cell fluids). Liquid methane and ammonia, twocommonly-suggested alternatives to water, are non-polar and lack many of water's usefulproperties. Thus there are sound reasons to believe life elsewhere will often be based oncarbon and water.

How Much Are You Worth?

You can buy the exact number and proportion of atoms to make a human for onlya few dollars. Likewise, you can buy the exact number and proportion of atoms to make aLexus from a junkyard for only a few dollars. If you want to talk about the exactarrangement of the atoms, the price goes up sharply. Buying the exact organic moleculesthat make up a human would cost millions of dollars, and that's only the chemicals we cansynthesize. There are many chemicals, like proteins, that we simply cannot synthesize asyet, let alone build a strand of DNA from scratch.

Alternatives to Carbon

Silicon makes up 27% of the earth's crust, as opposed to about 0.03% for carbon. So whatever is preventing us from finding silicon-based life, it's not scarcity. Why silicon? Because silicon is right below carbon in the Periodic Table, and like carbon, bonds to four neighboring atoms. But carbon easily does something that silicon does not do easily: bond to itself. Instead, silicon bonds to oxygen, which in turn bonds to another silicon, and so on. Silicon and oxygen (collectively called silica) form a dazzling array of complex structures, so that we can say silicon is to minerals what carbon is to life. Carbon can do another thing that heavier elements find very difficult: it can share more than one electron with a neighbor. These multiple bonds can serve to store energy or code information.

Silicon suffers from other chemical disadvantages. Carbon combined with oxygen makes a gas, which organisms can take in from the atmosphere. Carbon dioxide also dissolves in water. Both features make it easy to recycle carbon. Silicon plus oxygen, however, makes quartz, which is not soluble in water. There are silicon containing gases, and silicon analogues of hydrocarbons, but they react violently with water. So if there is silicon based life, it will be very different from carbon life, and its environment will have to be very different.

Return to Explorations of the Universe Index
Return to Professor Dutch's Home Page

Created 8 December 2008, Last Update 17 January 2020